34 



Since platinum contracts more on cooling than does quartz, there appears a 

 space between the platinum core and its quartz insulation of a size sufficient 

 to allow any electrolyte to enter by capillarity. The platinum tip is melted 

 to a bulb which by contraction of the cooling platinum perfectly seals the sur- 

 rounding space. A coat of platinum black is then deposited electrically on the 

 platinum tip and the electrode is cleaned in dilute HgSO^ and thoroughly washed 

 in hot distilled water. In order to control the gas content on the electrode 

 and protect it from contamination the electrode was mounted and used as follows; 



oo tkb «i mo> 



;t. i. 



A two-armed Pyrex glass shank is fitted by cement with two rubber plugs (Pig. 18, 

 A. r.p.) and filled with mercury. Opposed to each plug is a finely threaded thumb- 

 screw. Into the rubber of the straight arm of the shank has been cemented a section 

 of a small brass tube. This supports one end of the electrode (e.l.). The platinum 

 wire of the electrode is of sufficient length to extend back into the mercury of 

 the shank. To afford electrical contact with the mercury, a platinum wire (p.w. ) 

 is sealed in the wall of the upper arm of the shank. Onto the single end of the 

 shank is cemented a right-angled stop-cock (s.c). The tip of the pipette is then 

 sealed tight with hard paraffin and simultaneously the stop-cock is turned off. 

 To effect this, seal, a small ball of paraffin mounted on the tip of a quartz rod 

 is brought into contact with the tip of the electrode ( Pig. 18, B). A platinum 

 filament is used to melt the paraffin, The tip of the micropipette is pushed 

 into the melting paraffin through which the gas continues to bubble until the 

 stop-cock is closed. Thereupon the melted paraffin enters the tip by capillarity 

 and with cooling produces a gas tight seal. The device is then fixed in place 

 on the micromanipulator. 



Another micropipette is filled with saturated ZGL in agar, and so serves as a 

 "salt bridge". The tips of the two micropipettes are inserted into the cell, and 

 the platinum (hydrogen) electrode is moved forward by means of the thumb screw, 

 pushing out the paraffin plug. (Fig. 18, C). 



